The present invention pertains to a process for synthesizing large size uniform emulsion polymer particles made from polymeric seeds onto which is added monomer polymerized in a second stage. The resultant large polymer particles may be homogeneous but normally will be of a heterogeneous nature and are particularly useful for providing improved physical properties in coatings, plastics, and adhesives. Core/shell particles can be effectively produced. Heterogeneous core/shell polymer particles wherein the core can be a rigid polymer having a high Tg relative to a resilient elastomeric shell having a lower Tg can be conveniently produced in large size without experiencing emulsion instability.
Although seeded emulsion polymerization processes and particles have been suggested in the past, in such processes said particle surface coverage by the surfactants must be less than 100% surface coverage or less than critical-micelle-concentration (CMC). However, too little surfactant or considerably less than 100% surface coverage of the polymer seed particles introduces stability problems and produces considerable excessive coagulation. For example, U.S. Pat. No. 3,657,172 and U.S. Pat. No. 3,426,101 specifically provide for high surface tension in second stage processing as well as maintaining the concentration of surfactant well below the critical-micelle-concentration (CMC) to avoid 100% surfactant coverage of the seed. More specifically, U.S. Pat. No. 3,397,165 proposes that the core surfactant surface coverage be maintained below 70% theoretical surface coverage in the second monomer addition.
It now has been found that the second stage monomer addition can be carefully controlled wherein substantially all of the second stage monomer addition effectively adds to the seed particles whereby the seed particle can be substantially increased due to utilizing excess surfactant beyond CMC or at least about 105% seed surface coverage. The second stage monomer addition can be accurately controlled in accordance with the process of this invention by using a certain balance of non-ionic surfactant and anionic surfactant at a combined surfactant coverage of between about 105% and 1000% of the polymer seed surface to provide a grown composite particle where the second stage monomer has polymerized and completely associated itself with the first stage seed particle. The process further provides a method of controlling the surfactant in the second stage monomer polymerization wherein the balanced combination of surfactants provides at least about 105% seed surface coverage and preferably between 105% and 500% seed surface coverage.
Accordingly, a primary object of this invention is to provide a process for producing stabilized large size emulsion particles formed by polymerizing all of the second stage monomer addition onto the polymer seed in the presence of a controlled combination of non-ionic and anionic surfactants. A further object is to provide a process for controlling the level of second generation new particles and provide a stabilized interdispersion of heterogeneous composite particles or core/shell particles in combination with a predetermined level of new generation particles achieved by controlling the levels of total surfactant above 105% seed surface coverage as well as controlling the balance ratio of non-ionic to anionic surfactants.
These and other advantages will become more apparent by referring to the drawings and detailed description of this invention.